Nitrided article of ferrous character containing a case-toughening element combined with a balanced case-forming element



' n w m. 7,1933 a mum nuns rmcn, or 'nuzannrir, new annsnr; assranon ro, TEE mm- UNITED STATES- PATENT; oar-Ice "NATIONAL mcxnr. comrm, me, or raw roan, n.1, a conrona'rron or Dana'- wans nrrnnmn nnrronn or rnnnons cmao'rnr. 'comranmm A oas s-rename ELEMENT coimnmn wrrn a Bax-moan ones-roams nnmrnm:

No Drawing. Application filed, June 12,

This invention relates to improved nitrided articles of ferrous character containing a case-toughening element combined with a balanced case-forming element and to a process of manufacturing such improved nitrided articles, and more particularl to I an improved nitrided article constitute of steel or a ferrous alloy and containing a case-toughening element combined with .a balanced case-fbrming element and to a process of roducing such nitrided article.

It is well own that relatively soft steels may be provided with a hard surface by case hardenin them Heretofore, 'cases have been pro uced by carburizing thesurfaces of the articles and then hardening them by uenching. Articles hardened in this man-- er were subject to warpage or distortion which rendered-some of such articles unfit for use or necessitated finishing them mechanically after. treatment. Moreover, ar

'ticles which were case hardened with carbon, could only be made with hardness up to about 650 to 750 on the Brinell hardness scale.

For roducing greater surface hardnesses in stee s, nitridmg was pro'osed. As it is well known, the nitriding nitride hardeningor nitrogen hardening) processes are generally more expensive than the col ventional case hardening (carburizing and hardening) processes. Steels of special composition are required (alloy-steels) and inaddition; the nitrided articles .lacked reliability and reproducibility. Moreover, when j cases of very high hardness were roduced they were relatively brittle and o 11 they cracked in use under comparatively lowpressure or shocks in machinery parts. Although the art was struggling to provide imrovement-s in nitrided steels, which would tter meet the demands of commercial, industrial and practicaluse, such improved nitrided steels have not been provided.

In the United States Patents #1,487,554

1 and #1,649,398 to Fry,*'a process of nitriding steels and an article produced thereby are set forth which describe that aluminum is an essential element in the steel to be nitrided. The Fry procem essentially in- Patents ginal' layers of an extraordinarily high hardnessare acquired in" steel alloys which conb Fry as shown in rear. Serial no.

Phillips in a later United States Patent 5 ,083 points out j certain limitations and disa vantages of the Fry process and roduct and states that Frys product is decient in toughness and cohesiveness and that molybdenum is necessary in the reduction of asatisf'actory commercial nitrided steel. In addition, the Fry patent steel ,for.

contains other defects. For example, Fry states thzt bythe'process described in his #1,4s7,554 and #1,649,398) martain 0.52.0% aluminum and either separately or in any desired-co bination 0.5 to 4.0%

silicon,manganese,ni c el, chromium, molybi denum, tun sten, vanadium, titanium or zir-v conium an -up to 0.6% carbon. It is now known that aluminum is not-essential to the a production of high hardness in the marginal ayers of steel w nitriding processes such as heating in ammonia. Y

, Other investigators have published that en treated by theflordinary I nickel not only has a retarding action on the nitriding process but also ,a deleterious or unfavorable efiect on the nitrided steel. The

aforesaid efiects of nickel have been investigatedby Genie Civil, vol. 91, p. as, 60, 86, 472 1927- onatschefte,*vol. 4, p. 137 (1923 and in Stahl and'Eisen 43,127 (1923') and ergeson as shown in his article in the Transactions of the American Society for Steel Treating, vol. 16 p. (1929), etc. r

l has now been discovered that certain -metals, for instance, nickel, molybdenumchromium', vanadium, heretofore considered as equivalents of each other are not equivalent and interchangeable but have difierent and sometimes directly opposite characteristics.

s articles in Kruppsche Guillet as shown in his article in 3 with respect to the nitriding processes for v nitriding steels and the products thereof that some of the metallic elements heretofore considered necessary in steels for nitriding are unnecessary; that a substantial improvement and increase in of the cases of nitrided steels may e effected; that an increased nitrogen difiusion in the article may be obtained ;-that the highly important hardness-toughness relations and the depth of the case produced by nitriding may be controlled and that an improved, stable, reliable and reproducible nitriding steel can be produced by corporating a case-forming element or several of'such elements conjointly with a case-toughening element or several such elevments in certain relation to each other in a steel, ferrous alloy or other metal or alloy ca able of being case-hardened by nitriding.

case-forming element; is meant such an ementthat causes the steel to take up nitrogen in its surface or marginal layers in such amaniier that the hardness in the mar- .ginal layers" is increased very appreciably in the toughness of the case. The term toughness as used in this specification represents improved ductility combinedwith im- 1 proved strength and sometimes also combined with improved shock resistance.

According to the -'-present discovery, the

casesforming element may be an element such as chromium, alumlnum, or some other ele-' ment which is capable of causing in a steel or ferrous alloy, a relatively high rate of absorption of-nitrogen and a relatively low rate of nitrogen diffusion, and the case-"toughensome other element which is capable of imingelement mayibe nickel, molybdenum or parting to the case on a nitrided steel or ferrous alloy, an increased ductilitycombined with an I increased strength. The casetoughening element or elements are added to steel -or ferrousalloy in certain proportions with respect to the case-forming element or elements so as not only to counteract em- ,brittlin'g and weakening'efi'ect's: of ,ithe caseforming element or elements in the case on 1 the article but also to improve the structure and other: physical pro rties of the .case formed on the article. T e additions of casetoughening and case forming elements may be so controlled as to improve, at the'same time, the properties of the core of the article.

'The elementsmentioned hereinabove have as.

' a dominantcharacteristic either case-forming properties or" case-toughening properties. In some instances elements may have approximately equal case-forming and case.-tough-' 'ening properties. For instance, vanadium depending upon its conditions of application employed the case on the 'artlcle does not including. percentages present exhibits both case-forming and cas'e-tougheningproperties. In-percentages around 0.5% it shows marked case-forming properties.

Generally speaking, the present invention contemplates increasing the percentage of the case-toughening element or. elements as the percentage of the case-forming element individualv characteristics, the case-form-.

ing element and the percentage thereof added to the metals or alloys under treatment and the composition of the. steel or ferrous alloy to be nitrided and upon the properties desired the final product.

In the practice of the'invention, it'is pre-- ferred to use nickel and molybdenum con-1' jointly as the case-toughening elements because not-only tough cases can be provided on .the article but also improved core properties not attainable by the use of either of these elements separately can be obtained. It

is preferred to have the nickel addition equal or exceed one part of nickel to one part of molybdenum (by weight), but the benefits obtained from the nickel and molybdenum as case-toughening elements are not restricted to these preferred pro ortions as'is shown by the examples hereina er described.

such a percentage of case-toughenin ment that the ratio of the percentage ocasetolfghening element to the percentage of the case-forming element is about 0.8: 1 when improved casetoughness is desired without an appreciable decrease in maximum hard- Generally speaking, it is preferred to 1156 e eness. Ratios ofnot less than about 0.8: 1.0

and not more than about 5: 1 should be used, the ratio depending upon the cliaracterlstics of the case-forming element's added and upon the properties desired in the nitrrded steels.

The higher ratios of case-toughening element to case forming element are employed in those instances where the maximum case hardness desired becomes lower. In practice, it has been found that a ratio of 3:1 gives sat sfactory and useful results. When the prior art ratios of less than about 0.8: 1 are employed, practically no case-toughening is effected and the case on the article is defective and brittle. On the other hand, when the prior art ratios of higher than about 5: 1 are For a better understanding of the invencrease incase toughness is obtained when the .steel is mtrided: in a usual manner, v1z,-

I "The foregoing samples were nitrided for about'48 hrs. at about 975 F. with about /30% dissociation of ammonia.

The increase in toughness produced by the relatively large increases of the case-toughening element-nickel is accompanied by a decrease in the maximum case hardness (ob- 25 served ator near the surfaceof the articles) the amount of the decrease in the hardness and the increase in the case toughness being controllable by the amounts of the nickel added. 80

The foregoing example clearly demonstrates that the present invention provides the art with a process to control the hardnesstoughnes s relations of the cases of nitrided steels by means of balanced additions of caseforming elements and case-toughening elements as well as with an improved product. The hardness-toughness controLfeature is of special and practical v-im ortance and indus trial and' commercial va ue since-the adjust- 40 ment of case hardness and case toughness by tempering subsequent to quenching, which is effective in carbon case hardened steels, is not, as one skilled in theart knows, available in the case on nitrided steels. 1

- The marginal or surface layers of my im *proved nitrided articles will not only better resist cracking of the case but will withstand higher loads than heretofore and may be deformed further than hithertowithout era ck ing. These features may be shown and demonstratgd by the following table showing p I results of orsion and bend tests obtained p r g a this Mlwfifi the)%0rre sponding one in'whi'ch thecase-forming ele mentshave not f-been balanced against the upon steels E, F and G of Table 3. f

Tables/lrforsion test results mt eteelal lan d (specimens-34inch diameter-8 inches-long).

- .The foregoing samples were nitrided for about 48 hours at about 97 5 F. with about 25/30% dissociation of ammonia.- x

Each of the foregoing results is the average of 2 tests.

I Table 5.Ben i test results on steels E, F cml'G 35 inchx'? incheatested (specimens ,5 ir'whX 4 inch spay) Table 8. 5

I i (O Si tough Steelswth'smallamountso 888 Mnandcustomary impuritieatc m measured Steel gether with approximately 1% no mm by loadiu each of the case-forming elements -4 k onBrl Aland Cr,and-

10 crack case E Case-toughenigg elements as 101- 800 aoo lows: practi lynone' r 3.53%Niand%%Mo--i m "900.

16', G 6.07%Nia'nd%%Mo 72o 1,140

a Load in Deflection Steel lbs. to in inches to crack case case E (No added nickel) 2835 044 F 3.5% added nlckel) 4180 .000 v G (5% added nickel) 4675 067 The foregoing samples were nitrided for about/18 hours at about 975 F. with about 25/30% dissociation of ammonia.

Each 'of the foregoing results is the average'of 2 tests. F .Many tests in which the nitridedsurfaces have been removed to various depths before testing have indicatedthat these torsion test. j results and bend test results are typical of-.=

thebeneficial influences throughout the hard marginal layers (case) of the nitrided articles. 7

Ewqmple lvo. 3 .The invention may be illustrated further by adding to low carbon steel containing.

its.

Cr about 1.15% .(by weight).

-- V about 055% (by weight).

Ni about 1.63% (by eight).. Fe about balance.

case-toughenin'g-elements' shows the marked -superior tyj of' the former. Theadditi onof; the fiickel results in-a'n appreciable increase in the case toughness without appreciable loss in the maximum case hardness.

Thetorsion test results ich fo'llo'w demonstrate the ability to control the hardness toughness relations of the marginal'layers of nitrided articles-through nickel additions temperatures, periods and cycles used inni tion, the following specific and illustrative examples'are given Ewwmpl e No. 1 a

- 2% (by'weight which maybe a ut- 1% (by wei t) ofaluminum and about 1% (b weight .of chrom1- um, and about lei y weight) of case toughening. elements which may be about (b weight) at nickel and about ,472, y wei t)'o molybdenum The compos1- 15 tion of t e steel may be as follows :-i

C about 0.2% (by weight).

Si about 0.3% (bg weight).

Mn a ut 0.6% y weight). P'abo iib 0.02% by weight 8 about 0.08% 'yweight element).

about 1.07% (by weight) (ease-forming l ea 110' about 0.24% (by weight) (case-toughening element ening element .4

' Fe aboutb ance.

'The case-forming elements and the casetoughening elements may be incorporated the steel' n anyconvenient manner. It is preferr however, to add the aforesaid elements the steel while the latter is in a molten condition. After the addition to. and incorporation in the steel of said elements, the steel may be cast into ingots or the like and articles made therefrom. The articles may then be subjected to nitriding -in any approved or suitable manner, as one skilled in the art will readily understand. Preferably, however, the articles are heated in am-' monla at 900-1000 'de F. for a customary period of time. arious temperatures,

riods of time, etc., may be used and the fol-j owing table gives some of the-customary triding Table 1;

mum at a temperature between m me rm remote.-

wora rcnoiooam tromlhouru a fromlto'flhoun v iorlhournta.

. Rikl'deaa abomination by 1,200" r Nitride at about 1,260 r. (allowed by one temerature between 000' m 1,000" r. I

' Nitride'at a temperature between-900 and- 1, F. raduaily increase temperature to 1,150.F. with increase in heating rate as temperatnte'rises. End at 1,150" 9 x 55 erties are tabulated: of articles 'made-froma case-toughening elements.

In accordance with the invention a steel, for instance has incorporated therein about of case-forminglelements,

Steel Noallovaddltions Cr about 1.05%; (by weight) (case-form Ni about 1.54% (by weight) (case-tough- (steel B-Table-2) or case-forming elements J corres high' ardness (compare steels C and D,

madame",

For 1 to hours,

; If, the toughening elements, in

similar steel substantially devoid of the combination of thecaseFformingeIements and the nosemonotronpurities, and

eupomary im- B (at'ornear auxheeol article) I 4 case to crack B On! ease-lo mina' chums Oase-iormin; elements with inaumcient caae-wuehenha elements Balanced eaae-Iormln; elements and case-toughening elements (H? 1, 91% 0, 54%

D as

The foregoing samples were nitridedifor about 48 hoursat about 975,F. about 25/30% dissociation of-ammonia.' v

- The addition of the case-toughening elements (l of nickel combined with 34% molybdenum) together with 2% of case-' forming elements (1% aluminum and 1% case toughness without an appreciable change in maximum ease hardness as compared to a steel containing only'case-forming elements responding steel without the nickel treated in a correspondingmanner and will give better support to the case when subjected to heavy pressures. In addition, the case ofthe nickeloontaining steel is far superior to that of the nding steel without nickel. It has Table 2) and, at the same, time, it is much tougher and when deformed will not-breakas readily as the case'of the corresponding steel without nickel.

v The nickel-containing steel is therefore E'mample No. 2

amounts appreciably above about 2 per cent are added tosteels containing approximate- 1y 1% of each of the case-forming elements aluminum and chromium, a substantial in- 5 chromium) has resulted in' greatly improved adjusted to the case-forming elements 25/3075 dissociation of ammonia. .The test specimenswere inch diameter by 8 inches 'taining about 2% chromium m a steel con present. taining about 2% chromium and about 2 70 Table 6 of mckel, viz:

I mu 8 'r mm a nfi mg i 11% 2; A X61110 m0 I I "a s d ewe Mm arm I can 0 Cr v N! fi 'ffi l 3 m Steel 0 Or I Ni kg onBrlnelI quenttonitrldlng' I lug I Brine I lone um'onnwmntlaooa 'leoos" m .25 an v 2.01 eto m. -12 a: e is t a a I Y I I m The foregoingsamples were nitrigedbfior about 48 hours at about 900 F. wit a ut' The steels hsted in Table 6' were'mtnded about 48 hours at about '97 5 F. with about long. All results are from tests in duplicate.

Example o..-. One aspect of the invention may be further illustrated by'the following: If, to a. steel contpugheningelements, say onl about ,4 per,

taining small amolints of carbon and the customary impurities, thereis added about 1 per cent of a case-forming element, such as chromium, a'nd insuflicient amounts I of casecent'of molybdenum, the resu tant hardnesstoughness relations of the marginal layers of I nitrided articles made therefrom will not {be as good as those which-can be secured by bal; anced additions made according to this in vention. In this instance, the addition of about 1%. per cent nickel combined with the aforesaid alloy additions completes the proper amount of case-toughening elementsto gite a relatively high degree of toughness without an appreciable decrease in maximum case hardness; This illustrates one feature of the control of the hardness-toughness relations of nitrided articles; The following table illustrates'the advantages of this fea- I ture of the present invention:

' about 48-hours; at about 97 5 F.'with abou 25/30% dissociation of' ammonia.

Example 5 The efiectiveness and usefulness of nickel as a case-toughenmg element 1n nitrided steels and articles made therefrom, which improvement constitutes one aspect of this invention, maybe further illustrated by comparis'on ,of the results of tests of a steel contion has very much higher case toughness. I

withoutappreciable" loss in maximum case nardnes as co pared to the steel 0 'of'Table '8 with unb anced composition comprising case-toughening elements.

important aspects ofthis invention 1s e-forming elements without compensating I practicability of the [control of the hardnesstoughness relations of the marginal layers (cases) of nitrided steels and articles made I therefrom by balancing the additions of casef forming elements and the additions of case toughening elements. 4 Nickel is especially effective in the groupof case toughening elements which may be used for this purpose as is shown by the results of the experiments which follow, The following table hows a steel Q, with. no case-toughening element, and

steels S and T which'embody the principles of the present invention.

' Table 9 I Steels with about 1% A], m the: Maximum 2: my sml with small amounts of O 8 Mn one hardon and customary impurities, and nessmono- :5 w the lol1owlng nickel additions tron-BM mm 3 No added case-toughening element- 805. i 3.69 added nickel -.I 645 ooo. j 'r 5.1% added nmm .i no 4,740

The foregoing samples were nitrided for about $8 h0l11S flt;flb 0ut 97 5 F. with about 25/30% dissociation'of ammonia. I

As is shown inTable 9, by adjusting the relative proportions of the case-form1ng-.ele-..

ments and case-toughening elements in the steel (in thiscase-by varying the amounts of the case-toughening element nickel), it

' has been practicablev to control the-hardnesstoughness relations of the "marginal layers- 1 25 of the nitridedarticles with in relatively wide limits. With about 3.5 to 5 er cent added nickel, there was a very mar increase in the case toughnessjvnth a correspondmg decrease m-the max1mum case-hardness. The

- ments and case-toughening elements present.

amount of the increase in the case-toughnessand the decrease in the maximum case-hardness was dependent upon the proportions of nickel added, that is to say upon the adjustment'of the proportions of case-forming ele- In addition to the foregoing examples, the following illustrative examples are embodiments of'the present invention and are listed in a table for convenience. In Table 10, the percentage of case-forming element or elements and of case-toughening element or elements to be added to an appropriate steel or ferrous alloy are given.

/' Table 10 Case-forming elements Case-toughening elements Chromium Aluminum Vanadium Molybdenum Nickel s 1. our 2.0% 1. 1.% 0.25 5.0% v. 1. a 0. 1.5? 1.0 1. 2oz; 1- .5 o

au%- as% as% m. 1.5,,

It will be 'obs iarved that the present invention case-hardness and case-toughness and or ferrous alloy is desired with a nitrided case/having relatively high hardnessv and a relatively goodtoughness, the ratio of casetoughening element to case-forming element present in the steel or ferrous alloy is about 0.8 3 1. On the other hand, in the instances where a steel or ferrous alloy is desired with a nitrided case having a relatively good hard ness and a relatively high toughness, higher ratios of case-toughening element to caseforming element, of the order of about 3: 1

and about 5,: 1 are used. In no case, however,

' is a ratio eater. than about 5: 1- em loyed.

element must be balanced or controlled with respect to the case-forming element. ,By means of balancing (or controlling) the casetoughenin element to the case-forming element as escribed herein, the advantages, new and/or improved results set forth in de,-'

tail hereinabove may be obtained. In addition if casesof equal depth are desired the time of nitriding may be shortened-by the use i j of the present process from about 5% to 25% 4 or more. Moreover, with corresponding ni triding procedures, is possible toproduce' rovides nitrided. steels having controlled O a process b which such nitrided steels may be produce In the instances where a steel by use of the present invention cases deeper than those obtained in the conventional ni-' trided steels. a

In carrying the present invention into practice, various percentages (by weight) and/or combinations of the different casetoughening elements and case-forming elements may be used. Ingeneral, steels and ferrous alloys containing the following elements and percentagesmay be employed to carry the present invention into practice: aluminum 0% to about 5%; chromium 0% to about 25%; nickel 0% to about 16%;

molybdenum 0% to about 12%; and vanadi- 11m 0% to about 2%. The present invention U may be carried into practice with steels or.

ferrous alloys including cast iron, malleableized cast iron, and substantially carbon-free I iron alloys.

When 11011, steel or ferrous allo is referred to 1nv the specification and c aims, it

of customary elements such as silicon,=manganese, sulfur,'phosphorus, etc. are present. In the claims the expression an element of "is to be understood that the usual amounts the chromium type is meant to refer to case-.

forming elements as described herein, whereas the expression an element of-the nickel type is meant to refer to case-toughening elements as described herein.

The crack test applied to nitrided cases on steels and ferrous alloy articles refers to a test in which the nitrided caseis subjected to stress as by the ball of a Brinell hardness testing machine and the case is heard to crack. In practice, the occurrence of a sharp noise or crack denotes that mechanical discontinuity, such as a rupture, has developed in the nitrided case. By carrying the test slightly further, the crack can be confirmed by the use of a microscope, by'visual examination or by a bend test or by a torsion test. It will be noted that the present invention provides an improved article of manufacture It will be also noted that the present invention provides an im'provednitrided steel or the like havin relatively high hardness combined with re atively high toughness in which the case hassubstantial depthswithout any detrimental tendency to surface scaling and in'which the core or body of the steel has sufiicient hardness andtoughness to support the case even -under relatively severe loads or impacts. J

When in the claims a nitrided case is referred to as having a relatively high hardness .and relatively high? toughness or relatively high hardness combinedwith relatively high toughness, hardnesses and toughnesses of the order of those set forth in the foregoing tablesare meant.

While specific reference has been made to treatment in ammonia, the described benefits resultin from the employment of the principles (if the present invention, recesses and products described, are not limited as to the source of the nitrogen which is absorbed in tlli mprginal layers of the steel to form a case 1. An article of manufacture comprising a ferrous body having a tough nitrid'ed case, as nitrided, and containing a'plurality of case-toughening elements of the group comprising nickel and molybdenum balanced with a plurality of "case-forming elements of the group comprising chromium, aluminum and vanadium, the ratio of said case-tough ening elements to said case-forming elements being not less than about 0.8 1 and not greater than about 5:1, and the case forming elements being present to an extent of more than about 1.5% and less than about 4% and the case-toughening elementbeing present to an extent less than about 6%.

2. An article of manufacture comprising a ferrous body having a tough nitrided case, as nitrided, and containing about 1.5% to about 3.5% of nickel and about 0.25% to about 0.75% of molybdenum acting as case- 'toughenin g elements balanced with about 0.5% chromium, about 0.5% aluminum and I about 0.5% vanadium acting as case-forming elements, the ratio of said case-toughening elements to said case-forming elements being not less than about 0.811 andnot greater than about 5:1, and the case-forming elements being present to an extent of more than about 1.5% and less than about 4% and the case-toughening elements being present toan extent less than about 6%. v

3. An article of manufacture comprising a ferrous body having a tough nitrided case,

' as nitrided, and containing about 2% to about 5% of nickel, and a material percent to about 0.5% of molybdenum acting as case-toughening elements balanced with about 1% to about "2% chromium and a material percent to about 1% of aluminum acting as case-forming elements, the ratio of said case-toughene ing elements to said case-forming elements being not less than about 0.8: 1 and not great-.- or than about 5: 1, and the case-forming ele-,

.ments being present to an extent of more" than about 1.5% and less than about 4% and the case-toughening elements being present to an extent less than about 6%.

In testimony whereof, I have hereunto set 1 my hand.

HERBERT JAMES FRENCH. 

